Sustainable Façades

Green infrastructure in cities and urban environments is transforming the way we live today and the energy use in buildings is undeniably a blisteringly hot topic.

The concern voiced is obvious, as buildings represent roughly a third of primary energy use globally.

It is crucial that buildings are designed well to minimise energy use during occupancy and construction, but the majority of buildings already constructed will continue to be in use for many years regardless of energy performance. This may not be the issue it first appears.

In the modern curtain wall building, the building’s tasks are separated – a structural shell provides floors, walls and ceilings, while the curtain wall protects the interior space from the exterior environment and creates the external aesthetic of the building. This separation of roles creates adaptability, allowing the building’s façade to evolve over time to meet the changing needs of occupants and architects. Properly designed and implemented, this could allow a building’s performance to increase over time, and would help to extend its service life, thus mitigating the building’s embodied carbon.

Particular attention in future will be paid to the embodied carbon of the building and façade. This is because as the CO2 emissions of buildings decrease with improving energy efficiency of equipment, the relative proportion of lifetime CO2 emissions due to the construction of the building increases.

Contemporary materials in commercial building construction

The most commonly used materials in commercial buildings include steel, aluminium, concrete, glass and some brick.

Steel came to replace the more brittle iron, both of which were favoured as they allowed greater spans than masonry. The use of steel has continued to be popular as buildings have grown larger, due to its strength, resistance to degradation, and its ease of use in construction.

Concrete is used in conjunction with steel to make reinforced sections and to add fireproofing to steel structures, but on façades it is often used on its own to provide supporting structures. Concrete is popular for its durability with little maintenance, its ability to be made into non-linear shapes and for its uniform aesthetic.

Glass is one of the most aesthetically pleasing façade building materials. It is relatively lightweight, is available in many sizes, and until recently, was the only commercially available material that allowed clear viewing. Aluminium is more commonly used to frame glazing, but is also gaining popularity in façade support structures. This is because aluminium is lightweight, recyclable without loss of performance, durable with little maintenance, comes in many profiles, and is easily workable.

Despite the wide variety of materials developed and used in other industries, the construction industry is somewhat conservative. This has meant that the shift in material choice has occurred more slowly when compared with that of the aerospace industry, or other smaller industries. Due to the technological conservatism of the construction industry, there is also relatively little variation of building material choice to suit local climates.

The longevity of façades

Compared to other products, buildings have a long service life. This may appear troublesome when new building techniques make older constructions seem energy inefficient and thermally obsolete. However, it is the façade that largely determines the energy performance of the building. Due to modern construction methods, the façade’s service life is not necessarily tied to the service life of the building. This is because commercial buildings are often built of a structural core, and a façade that needs only to support its own weight. This allows the façade to adapt as technologies improve without having to demolish and rebuild the entire structure.

Embodied carbon of façade materials

The embodied carbon of a material is used to describe the CO2 emissions that result from the manufacture of a particular product, due to factors such as energy use, transportation and resource extraction. Some materials with high embodied carbon may be serviceable for many years or have good thermal performance, whereas others may have low embodied, but a short service life, or poor thermal performance. When trying to weigh up the benefits of each system, as is often the case, the overall carbon balance may have to be revealed through careful calculation.

For some materials, the embodied carbon can form most of the total carbon footprint, for others the embodied carbon can be neutral or even negative. For example, cement, essential to concrete, emits CO2 when heated during the manufacturing process. This results in nearly half of the carbon emissions of cement not being due to energy use, but to chemical reactions during manufacture.

At the other end of the scale, trees take up CO2 to build wooden structures. By harvesting this wood mass and incorporating it into the built environment, CO2 is removed from the atmosphere and stored for as long as the building stands. This presents the opportunity to have a building material that is carbon neutral or negative for the duration of the building’s life. The materials may remain carbon neutral or negative for even longer if, when the building reaches end of life, the materials are removed and reused as part of deconstruction as opposed to demolition.